With reheating-type plants of related single shaft combined cycle plants, such as disclosed in, for example, Japanese Patent Laid-open Publication No. Sho. 62-101809, steam provided to a high-pressure turbine bypasses the high-pressure turbine and is supplied to the reheating turbine. The high-pressure turbine is then made to communicate with a condenser and heating accompanying windage loss of high-pressure turbines and reheating turbines at the time of activation of the steam turbine is prevented.